Esterification of the hydroxyl groups in the cellulose, hemicellulose, and lignin in wood provides lumber produced therefrom with improved dimensional stability, as well as resistance to attack from biological organisms. Acetylation is one method for esterifying the components of wood. For brevity herein, this process will be referred to as acetylation of wood.
One method for acetylating wood reacts acetic anhydride with hydroxyl groups in the wood to provide ester groups through a substitution reaction. Since acetic anhydride can also react with water present in wood to produce acetic acid, the wood is typically dried prior to acetylation. Conventional methods of drying wood include kiln drying with steam. Kiln drying can degrade the wood by causing warping, checking, and splitting, and requires a significant time to reach the desired moisture level.
After kiln drying, the wood is transported from the kiln to a reaction vessel. While in transit, the wood may absorb additional moisture. Dried wood is next impregnated with acetic anhydride, and heat is applied to initiate the exothermic acetylation reaction between the hydroxyl groups and the acetyl groups. Uniform temperature control is desirable to provide evenly distributed ester groups throughout the wood. Convective heating is one option for heating the wood but suffers from the disadvantage of taking a long period of time to heat the wood through convection to the desired temperature. Another technique for applying heat during the acetylation reaction is with the use of microwave radiation.
Conventional acetylation methods have typically employed small wood pieces. Larger wood pieces are more limited in their ability to become impregnated with and be stripped of chemicals. Therefore, it is difficult or commercially unfeasible to produce solid lumber products made from whole acetylated wood because even or uniform acetylation cannot be achieved.
While some improvements have occurred in acetylation, there still remains a need to provide methods that increase the efficiency and reduce labor and costs associated with the acetylation of wood with acetic anhydride.
The present invention fulfills this need and has further related advantages as specified below.